The present invention relates to a surface acoustic wave device, and a communication device employing the surface acoustic wave device. More particularly, the present invention relates to a spread spectrum (hereinafter abbreviated to "SS") communication system and a communication device suitable for a SS communication system.
Recently, an SS communication system for communication devices, having high noise resistance and excellent privacy function has become an object of attention.
Generally, in the SS communication system, an SS signal, i.e., a transmit signal, is a signal spread in a very wide frequency band produced by the SS-modulation of a carrier wave in a narrow frequency band, obtained by modulating information to be transferred by a base band signal, using a predetermined code series having a predetermined high bit rate.
When receiving a signal which has been SS-modulated by a direct modulation system using, for example, a pseudo noise (hereinafter abbreviated to "PN") code series, in such an SS communication system, a demodulator for demodulating the SS signal uses the same PN code as the PN code used by the transmitting device for SS-modulation, and extracts the received SS signal as an information bit signal only when the received SS signal coincides with the PN code.
The SS signal which has been SS-modulated by using such a given code has a frequency band which is far wider then the frequency band for an ordinary communication system, and hence the SS signal is resistant to noise, has a low power spectrum density and an excellent privacy function. Therefore, the SS signal is difficult to wiretap. Using a predetermined code, such as a PN code, for SS-modulation and SS-demodulation, the SS communication system need not take frequency assignment into consideration, whereas the ordinary communication system needs to assign frequencies to avoid interference. Therefore, the SS communication system is not subject to a frequency shortage problem in assigning frequencies to communication stations due to increase in communication stations.
However, communication devices of the SS communication system usually are very costly because these communication devices need a large-scale integrated circuit and, therefore, the use of these communication devices has been limited to military communication or satellite communication.
A demodulator for demodulating an SS signal, employing a surface acoustic wave device as a matched filter and a delaying circuit for delaying an output correlation peak of the matched filter by one period of a PN code, proposed in, for example, Japanese Patent Laid-open (Kokai) No. 4-346528, is a communication device of the SS system having a simple configuration, and is capable of providing for being fabricated at a low cost and of high-speed signal processing.
A split-connect type surface acoustic wave device mentioned in "1973 ULTRASONICS SYMPOSIUM Proceedings", pp. 407-409, is employed as a surface acoustic wave device in such a demodulator. This surface acoustic wave device has an electrode line width and an electrode gap width equal to 1/8 of the wavelength of a surface wave of a frequency corresponding to the center frequency of the filter and pairs of electrode digits alternately connected to electrodes of different polarities, to suppress mass-electrical-loading (hereinafter abbreviated to "MEL") due to the difference in acoustic impedance to surface waves between portions provided with electrodes and those not provided with electrodes.
SS signals having frequencies in a frequency band of 100 to 150 MHz are preferable for this known communication device. When SS communication is performed by using SS signals having frequencies in a frequency band including, for example, a center frequency of 2.4 GHz and which is higher than or equal to the UHF band, the GHz band frequency of an SS signal is reduced to a frequency in a low frequency band, for example, a MHz band, the SS signal is converted into a correlation output with a PN code by a matched filter 35, the correlation output is multiplied by a signal provided one bit before the correlation output by a delaying circuit 36 using a mixer 37 for demodulation, and the waveform of the output signal of the mixer 37 is converted into a digital rectangular waveform by a hold waveform shaping circuit 38, as shown in FIG. 14, because a surface acoustic wave device serving as the matched filter 35, and a surface acoustic wave device serving as the delaying circuit 36 are unable to deal with signals of frequencies in a frequency band higher than or equal to the UHF band. Therefore, the communication device that uses signals of frequencies in a frequency band higher than or equal to the UHF band needs a down-converter 33 for reducing the frequencies of the SS signals.
The down-converter 33 needs an oscillator, which increases the cost and hampers miniaturization. In addition, in a communication device provided with a modulator and a demodulator, the down-converter 33 becomes a source of interference when the modulator performs SS-modulation.
If the surface acoustic wave device being used as a matched filter is able to deal with signals having frequencies higher than or equal to those in the UHF band, the down-converter 33 becomes unnecessary. However, the electrodes of the conventional surface acoustic wave device have a small electrode line width when the center frequency of the filter is high. For example, the electrode line width must be 0.2 .mu.m or below to make a filter having a center frequency of 2.4 GHz. However, it is difficult to form electrode lines of such a small electrode line width using conventional photolithography. If a solid type electrode structure in which alternate electrode digits are connected to common electrodes of different polarities, is employed, the electrode line width may be doubled, and can be formed by conventional photolithography. However, MEL occurs to deteriorate the filtering characteristics resulting in the deterioration of the performance of the communication device.